From treating patients by dialysis, devices are known for heating dialysis fluid before the fluid flows into the dialyzer or blood treatment filter in which substances, e.g. blood and dialysis solutions, are exchanged through a membrane which usually is of a semi-permeable type. Some of these heating devices require achieving a secure contact or require keeping a maximum distance between the heating device which may, for example, be shaped as heating coils or heating spindles and a container containing the dialysis fluid to be heated, wherein the container is surrounded by said heating device and wherein the container may, by way of example, be embodied as a bag. With a heating device of this type, heating the dialysis fluid as intended depends on how well the container contacts the heating device, which may, for example, be embodied as a bag heater (or vice versa). This applies to other fluids which are heated in containers as well.